1. Field of the Invention
The present invention relates generally to the field of hypercalciuria. More particularly, it concerns the determination and identification of a genetic basis for absorptive hypercalciuria and osteoporosis with hypercalciuria. This determination allows the development of diagnostics and therapeutics.
2. Description of Related Art
Nephrolithiasis is a common debilitating clinical disorder associated with an estimated lifetime risk of stone formation of 20% for males and of 5% for females in the western population. In the United States, the annual incidence of nephrolithiasis is 7 to 21 per 10,000, with up to 10% of patients undergoing active stone passage requiring hospitalization to control complications.
Absorptive hypercalciuria (AH) causes stone formation in about 50% of the reported cases. AH is invariably associated with intestinal hyperabsorption of calcium in the presence of normal serum calcium concentration and a normal or suppressed level of parathyroid hormone. Osteoporosis or bone loss, particularly of trabecular bone (Barkin et al., 1985), is a frequent complication. The mechanism by which hypercalciuria leads to osteoporosis is not fully understood.
Both clinical and experimental data indicate that AH is heterogeneous in origin. Serum calcitriol concentration is high in some, but not all, patients with AH (Kaplan et al., 1977, Broadus et al., 1984, Breslau et al., 1992). The reduced calcitriol synthesis with ketoconazole restores normal intestinal calcium absorption in some patients, but not in all of them (Breslau et al., 1992). While spinal bone density is often low, some patients enjoy normal density. Also some patients with AH exhibit exaggerated renal synthesis of 1,25(OH)2D (Insogna et al., 1985).
One mechanism that has been proposed for the basis of AH involves an increase in the number of vitamin D receptors in the intestine. Li et al. demonstrated such an increase in the intestine of a normocalcemic, normal calcitriolemic rat model for AH (Li et al., 1993). An elevated level of vitamin D receptors was also observed in the activated blood lymphocytes of some AH patients who had normal levels of circulating 1,25 (OH)2D (Zerwekh et al., 1993). While evidence of genetic linkage between AH and the vitamin D receptor or the 1, α-hydroxylase gene loci has been pursued, none has been produced to suggest any linkage. Other reports implicate vitamin D-independent factors. A ketoconazole study showed some patients to be ketoconazole-resistant because their intestinal hyperabsorption of calcium and hypercalciuria were unaffected by treatment (Breslau et al., 1992). Bianchi et al. suggested that an activation of the plasma membrane of Ca/ATPase may be etiologically important in AH, based on the finding of accelerated activity of this enzyme in red blood cells (Bianchi et al., 1988).
While environmental-nutritional factors have been implicated in the pathogenesis of AH (Hess et al., 1993), strong evidence suggests involvement of a genetic process in AH; a familial pattern is present in 45% or reported stone cases, and an autosomal dominant inheritance pattern has been disclosed (Coe et al., 1979, Pak et al., 1981). Stone formation may be influenced by multiple risk factors, both environmental and intrinsic. However, the intrinsic factors, that is, a molecular and genetic basis of AH, have not yet been characterized. The evaluation of large stone-forming kindreds by Coe et al. (1979) and by the group (Pak et al., 1981) indicated that AH was inherited in an autosomal dominant manner. However, no molecular genetic basis for the intestinal hyperabsorption of calcium in AH has been identified. It has been speculated that AH could result from stimulation of renal 1,25(OH)2D synthesis, (Insogna et al., 1985; Broadus et al., 1984) increased vitamin D receptor sensitivity, (Breslau et al., 1992; Li et al., 1993; Zerwekh et al., 1993) or activation of the plasma membrane Ca/ATPase (Bianchi et al., 1988). The prior studies failed to show an abnormal vitamin D receptor genotype (Zerwekh et al., 1995) or a positive linkage between AH and gene loci expected to be involved vitamin D metabolism (Reed et al., 1996). In Dent's disease and related conditions that have a clinical presentation that includes hypercalciuria and nephrolithiasis, a mutation in the chloride transporter gene, CLCN5, has been reported (Lloyd et al., 1996). However, AH, unlike Dent's Disease, does not have an X-linked mode of inheritance.
There has been a clear need to identify, if any, a genetic basis for AH. Such information would yield a better understanding of the condition, providing important implications in the diagnosis and therapy of AH. Furthermore, identification of a genetic basis for AH may furnish a definitive diagnostic test to identify at risk, but asymptomatic, individuals. Detection of such individuals then allows for dietary and therapeutic intervention to prevent the onset of stone disease or osteoporosis.
AH is frequently accompanied by bone loss or “osteoporosis”. Osteoporosis is defined as a group of disorders that is characterized by aberrant bone remodeling; the net rate of bone resorption is greater, rather than in dynamic equilibrium with, the rate of bone formation. The condition can occur as either a primary disorder or as a disorder associated with a various disease, such as hypercalciuria. Examples of osteoporosis with hypercalciuria include ideopathic osteoporosis with hypercalciuria and postmenopausal osteoporosis with hypercalciuria. Ideopathic ostoeporosis is often times seen in young women or men demonstrating increased calcium absorption for unknown reasons. Postmenopausal osteoporosis is seen in postmenopausal women and is associated with decreased estrogen levels and increased calcium absorption. L2-L4 bone density was 10% below normal levels overall and had declined by more than 25% in approximately one-fourth of patients who had AH (Pietchmann et al., 1992). Histomorphometric studies confirmed an abnormal bone picture, characterized by an increased osteoclastic resorption surface (Bordier et al., 1977), decreased osteoblastic activity (Malluche et al., 1980), or both (Steiniche et al., 1989) in idiopathic hypercalciuria. In the animal model of AH, bone calcium loss was associated with an increase in Vitamin D receptor (Krieger et al., 1996). However, the exact role of bone in the pathogenesis of AH remains unclear. Some have implicated cytokine involvement in the etiology of bone loss associated with AH (Pacifici et al., 1990; Weisinger et al., 1996). Increased production of IL-1 by monocytes of hypercalciuric patients has been observed (Pacifici et al., 1990), and a recent study reported that hypercalciuric patients with stones had increased levels of basal secretion of IL-1α by circulating monocytes and enhanced levels of TNFα and L-6 production by activated monocytes (Weisinger et al., 1996). It was suggested that IL-1 could stimulate prostaglandin production, which would account for previous reports of high PGE2 in hypercalciuric stone-formers (Buck et al., 1981). Weisinger subsequently implied that prostaglandin-dependent synthesis of calcitriol could cause hyperabsorption of calcium and hypercalciuria, but no concrete evidence of this has been produced.
An understanding of a molecular/genetic basis of AH would facilitate the development of new therapeutic strategies for the treatment of AH. Current diagnosis is based on stone risk profile, markers for bone turnover, and bone densitometry and also involves blood tests and urinalysis. The results of many of these tests are influenced significantly by diet and thus require patient compliance with defined diet. A straightforward genetic test would eliminate the complications of extended testing and increase the certainty of the diagnosis. Unfortunately such a test is not presently available.